Lithium-ion cells and batteries are secondary (i.e., rechargeable) energy storage devices well known in the art. The lithium-ion cell, known also as a rocking chair type lithium battery, typically comprises a carbonaceous negative electrode that is capable of intercalating lithium-ions, a lithium-retentive positive electrode that is also capable of intercalating lithium-ions, and a separator impregnated with non-aqueous, lithium-ion-conducting electrolyte therebetween.
The negative carbon electrode comprises any of the various types of carbon (e.g., graphite, coke, mesophase carbon, carbon fiber, etc.) which are capable of reversibly storing lithium species, and which are bonded to an electrically conductive current collector (e.g., copper foil) by means of a suitable organic binder (e.g., polyvinylidene difluoride (PVDF), polyethylene (PE), polypropylene (PP), polyvinylidene difluoride-hexafluoropropylene copolymer (PVDF-HFP), etc.
The positive electrode comprises such materials as transition metal chalcogenides that are bonded to an electrically conductive current collector (e.g., aluminum foil) by a suitable organic binder. Chalcogenide compounds include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. Lithiated transition metal oxides are at present the preferred positive electrode intercalation compounds. Examples of suitable cathode materials include LiMn2O4, LiCoO2 and LiNiO2, their solid solutions and/or their combination with other metal oxides.
The electrolyte in such lithium-ion cells comprises a lithium salt dissolved in a non-aqueous solvent which may be (1) completely liquid, (2) an immobilized liquid, (e.g., gelled or entrapped in a polymer matrix), or (3) a pure polymer. Known polymer matrices for entrapping the electrolyte include polyacrylates, polyurethanes, polydialkylsiloxanes, polymethacrylates, polyphosphazenes, polyethers, polyfluorides and polycarbonates. Known polymers for pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEO), or polyphosphazenes (PPE). Known lithium salts for this purpose include, for example, LiPF6, LiClO4, LiSCN, LiAlCl4, LiBF4, LiN(CF3SO2)2, LiCF3SO3, LiC(SO2CF3)3, LiO3SCF2CF3, LiC6F5SO3, LiO2CF3, LiAsF6, and LiSbF6. Known organic solvents for the lithium salts include, for example, alkylcarbonates (e.g., propylene carbonate, ethylene carbonate), dialkyl carbonates, cyclic ethers, cyclic esters, glymes, lactones, formates, esters, sulfones, nitrites, and oxazolidinones. The electrolyte is incorporated into the pores of the positive and negative electrode and in a separator layer between the positive and negative electrode. The separator may be a porous polymer material such as polyethylene, polyfluoride, polypropylene or polyurethane, or may be glass material, for example, containing a small percentage of a polymeric material, or may be any other suitable ceramic or ceramic/polymer material.
Lithium-ion cells made from pure polymer electrolytes, or liquid electrolytes entrapped in a polymer matrix, are known in the art as “lithium-ion polymer” cells, and the electrolytes therefore are known as polymeric electrolytes. Lithium-polymer cells are often made by laminating thin films of the negative electrode, positive electrode and separator together wherein the separator layer is sandwiched between the negative electrode and positive electrode layers to form an individual cell, and a plurality of such cells are bundled together to form a higher energy/voltage battery.
Formation and handling of the thin film electrodes presents a challenge for battery cell manufacturers. Generally, the precursor materials are formed as a slurry containing the electrode active material in powder form, a polymeric binder and a solvent. The slurry is then cast or coated onto a temporary substrate (e.g., Mylar or paper) or onto a current collector (e.g., an aluminum foil) to provide a sheet form, and then dried to remove the solvent. Various coating means, including spraying, spin-coating blade-coating, electrostatic spraying, painting and the like, may be used. The sheet of precursor material may be subsequently calendered, if necessary, to reduce the sheet thickness and/or to densify the active material prior to lamination. Alternatively, the precursor materials may be extruded into sheet form, or otherwise processed to produce a precursor sheet or film of suitable thickness that may then be laminated to the other cell components. If the electrode precursor sheet does not have suitable mechanical properties, such as sufficient strength, the sheet will have a tendency to collapse upon itself or become otherwise damaged, either during casting, calendaring or transfer to a laminating station, making it unsuitable for use in a battery cell.
Fluoropolymers, such as polyvinylidene difluoride (PVDF), have typically been used for the polymer binders due to their electrochemical and chemical inactivity in relation to most polymer, gel or liquid electrolytes. Under the conditions that these fluoropolymers do not produce free-standing electrode sheets at typical binder contents of 2-10 wt. %, one possible solution is to increase the binder content, but this requires an equivalent decrease in active material content. High active material loading is desirable to achieve a high energy density, such that increasing the binder content results in an undesirable decrease in the energy density of the battery cell.
There is thus a need to develop an electrode precursor material that may be formed into a free-standing electrode sheet while achieving a desirably high energy density.